Unusually clear skies this weekend (see forecast) offer Marylanders who wander outdoors after sunset a nice opportunity to watch for the re-appearance of the moon following its dark, "New Moon" phase for April. Here's the deal:

The moon reaches its "new" phase at 3:44 p.m. today. That's when it is directly between the Earth and the sun. We can't actually see a "new" moon unless the alignment is nearly perfect, in which case we observe a total or partial solar eclipse. Most months, the moon is just too close to the sun without blocking its light. The solar glare - and the fact that the side of the moon facing us is not illuminated by the sun's light - prevents us from seeing it at all.

That begins to change as the moon, continuing on its orbit from day to day, gets far enough east of the sun that we begin to see sunlight reflected off a slim crescent of the moon's surface.

This graceful lunar crescent is always beautiful to see, and especially so when it is unusually slender. And that requires both sharp eyes and clear skies. Which brings us to this weekend.

On Friday evening, only 1 percent of the moon's disk, as seen from Earth, will be illuminated and reflected to our eyes. Look low in the west after the sun has set. That occurs just before 8 p.m. Binoculars might help you spot it.

On Saturday, the task gets easier. The moon will have moved farther along toward the east, farther from the setting sun. Fully 5 percent of its disk will be reflecting sunlight, and spotting it should be fairly easy if skies remain clear. Each night after that the moon moves farther eastward and more of it becomes illuminated.

One of the most pleasing sights associated with these "young" moons is "Earthshine." That's the light reaching us from the rest of the moon's disk - that portion (from our perspective) that's not illuminated directly by the sun. Instead, it is bathed in relatively dim, filtered sunlight that's been bounced first off the "day" side of the Earth. So, the light's path goes from the sun, to the Earth, to the moon, then back to our eyes.

Astronomers working in the field of "astrobiology" have been using Earthshine to prepare for the day when we will be studying the light reflected from planets circling other sun-like stars in a search for for signs of life. By analyzing the sunlight reflected from the Earth to the moon and back, they hope to learn how that reflected light is altered as it's bounced off our planet. From that, they want to learn which of those alterations are caused by chemical processes unique to life. With that knowledge in their tool box, they hope one day to be better able to detect the presence of life on alien worlds orbiting distant "suns."